How to organize a modern photometric laboratory Part 1 – selecting a goniometer system

Many manufacturers of lighting products use external measurement laboratories to verify compliance with standards and assess performance. Some manufacturers or importers rely solely on their suppliers’ datasheets and try to avoid having to carry out measurements on their own. This situation is quite common not only among small start-ups but also larger lighting companies in many European countries and beyond.

This situation is due to the low awareness of how much the parameters of the components differ from the functional parameters of the finished lighting product. Apart from that, paradoxically, many times higher capital expenditures are made on the organization and equipment of production departments, and it is difficult to find appropriate funds to finance the implementation of quality control while at the same time manufacturers find it impossible to verify what is actually delivered to customers.

Published by Miko Przybyla in LED Lighting Magazine

Why it is worthwhile to have an in-house laboratory?

This trend has recently changed and the need for in-house verification of product quality and laboratory or in-house quality control within the company is increasing. This is due to the increasing competition in the market and the need to optimize products and look for unique technological solutions. Moreover, with the multitude of different products in the manufacturer’s catalogue, sending luminaires and components for testing to an external laboratory becomes time-consuming  and even too expensive. Regardless of the size of the company and its business profile, more and more companies decide to build their own photometric laboratory in order to verify the quality of their products, compare them with the competition and better select quality components purchased from different suppliers. This approach has many advantages: it supports the sustainable development of the company and strengthens its market position. Companies with their own measurement facilities make fewer mistakes at the design and product introduction stage, can better select components and suppliers. In addition, companies are able to react more quickly to changing technology, the know-how remains within the company and a lasting advantage over the competition is built, and the company’s value increases.

Let us begin the discussion on the organization of an in-house photometric measurement laboratory with the proper selection of an appropriate goniometric system. What types of goniometers are available and which ones should be used in the measurement of lamps and luminaires for general lighting purposes, and which ones in the measurement of technical lamps and illuminators? What are the ways of measurement and what values can be measured? In addition, we will analyze the measuring equipment necessary specifically for the characterization of LED products. We will also describe the requirements and suggestions for the preparation of the laboratory room.

Types of goniometers used in photometry

Different types of goniometers are available on the market; they are distinguished by their geometry of measurement, i.e. how photometric data are collected and how the measuring system is implemented in a mechanical sense.

Types of goniometers can be basically divided into groups 1, 2 and 3, described in detail in CIE 70 of 1987 [1]. These types are also called A, B and C and differ in the way the luminaire is rotated during measurement and in the system of photometric data obtained during such measurements.

 

Schematic drawings and a description of the different types are shown in Figure 1.

C type Goniometer System

B type Goniometer System

A type Goniometer System

For photometric measurements of products intended for general lighting purposes it is recommended to use a type C goniometer. Goniometers A or B, on the other hand, are most often used in the measurement of car lamps, projectors and signal lamps in land transport and aviation.

It is worth mentioning that there are also different designs of C-type goniometers. When measuring gas discharge lamps, the position during measurements must be identical to the working position of the luminaire, because the change in the position of the light source significantly changes the level of the light output – lumen (total luminous flux). For this purpose, goniometers with movable measuring arm or goniometers with mirror modes are used. Both of these solutions are very expensive and require very large laboratory rooms in order to ensure a suitable minimum distance for photometering – which will be discussed further below.

When measuring LED lamps and luminaires, goniometers rotating the luminaire are most frequently used, and the measuring device is located at a fixed, sufficiently distant point. The operating position of a light-emitting diode does not fundamentally affect its performance and efficacy. There are only differences due to the different heat dissipation by the radiator, which can result in a decrease in light output. However, the differences in the readings of the luminaire’s flux values depending on the position of the luminaire are usually just over 1 to 3% and can be compensated by using an appropriate measurement procedure and corrections made to the measurement results in accordance with the recommendations of international recognized standard CIE S025 E:2015.

Far-field goniometers versus near field goniometers

All goniophotometer are used primarily to collect photometric data, i.e. the so-called LID diagrams (Luminous Intensity Distribution) that represent the spatial distribution of light intensity [cd] of the source. These data characterize a luminaire or a source and are necessary for the purpose of lighting projection with the use of light design software.

The straight forward way to calculate the value of light intensity [cd] is using the lux values and distance, therefore most often far-field goniometers are used. Such a measuring system assumes the placement of a measuring device distant from the source at such a distance at which the shape and dimensions of the luminaire are not important and it is measured as a point source. The appropriate distance is the so-called minimum photometric distance, i.e. the distance for which the inverse of the square law is true. When determining the minimum photometric distance, it is advisable to follow the recommendations of CIE standards, which suggest a distance of 5, 10 or 15 times the size of the luminaire’s illuminating surface. Thus, with a luminaire size of 1 m, the minimum distance should be 5 m for luminaires with wide distribution and soft LID shape. For luminaires with narrow distribution, measurements from a distance of 10 m are suggested, and for specific luminaires with directional distribution measurements from a distance of 15 m are suggested (Fig. 2).

Fig. 2. The photometeric distance depends on the size of the luminaire.

For luminous intensity calculations based on luminance [cd/m2] of the luminaire near-field goniometers are used, which most often employ imaging luminance meters located at a short distance from the tested source; they measure the luminance of the illuminating surface at different angles. Such a solution is currently not recommended for the measurement of absolute photometric values in luminaires and is most often used in the case of testing displays or technical illuminators.

Modern implementations of goniometric systems will use fully programmable, computer-controlled mechanical-electronic systems. The use of servo motors with absolute encoders in the drive enables precise control and reconstruction of the goniometer position. In addition to the control of both the C and gamma axes, systems with motorised Z axes are also available, allowing for convenient positioning of different luminaires of different sizes in the measuring axis. Advanced swivel connectors allow the power supply and measuring circuits to be supplied directly near the mounting plate without the risk of entanglement of the power supply cables when the luminaire is rotated. The figures below show a type C-type goniometer with three mechanized axes and a schematic diagram of how to create goniometric files for a typical luminaire (Fig. 3).

 

Fig. 3. Type C goniometer with three mechanized axes and schematic diagram of creating goniometric files for a typical luminaire.

 

Preparation of the room

As it is easy to notice, the size of the tested luminaire with far-field goniometers recommended for measurements of luminaires has a significant impact on the size and location of the laboratory room. When deciding on the location, the minimum working area of the goniometer, the length of the room and the space for the operator should be taken into account. A working area of 2.5 x 2.5 m should be provided for a typical 1.5 m or 1.8 m long goniometer for luminaires. The length of the room, depending on the size of the tested luminaires, should be from 7 to 10 m, and in some cases even 15 m. When planning the location of a room, it can be predicted that the distance between the luminaire and the device on a tripod will usually be 10 m. When measuring larger luminaires, which are rarely measured, if possible, we move the tripod with the measuring device to an adjacent room or set it up in a corridor. Such solutions work well in many large companies and even accredited laboratories all over the world.

Of course, according to the recommendations of the standards, the measuring room should be dark, without access to daylight (during measurements) – walls, ceiling and floor covered with material or paint with a low reflection index (black or graphite). The room should be free of dust, vibrations and a stable temperature of 25°C and air movement should not exceed 0.25 m/s.

Professional goniometric systems are equipped with a measuring head with a tube limiting the so-called stray light, i.e. unwanted light reflected from elements and surfaces of the laboratory, which could be measured by the measuring system. Thanks to this solution, no additional partitions ( baffles ) between the luminaire and the measuring device are required. The tube is equipped with specially designed elements limiting the influence of diffuse light and limiting the viewing angle of the measuring head. In practice, this means that it is possible to work in the darkroom at the operator’s workstation with illuminated workstation and the blackening can be limited to the work area of the goniometer and the surface directly behind the goniometer.

When choosing a goniometric system supplier, it is worth paying attention to the technical support that can be provided by the manufacturer. Many important decisions should be made well before the purchase of measuring equipment. Moreover, at the installation and training stage, many questions arise, especially from people who will be trained in the operation of the system. It often happens that they do not have any experience in photometric measurements. During the use of the equipment, there may also be many new questions or issues to be solved, and it is worth having the opportunity to receive appropriate support and possibly training from the manufacturer.

Special requirements for LED measurement

LED measurement is not only related to photometric values such as light distribution curves [cd] or the calculation of the total luminous flux [lm]. For the full characteristics of LED products it is necessary to ensure appropriate power supply conditions and measurements of electrical quantities [2]. Therefore, with such a comprehensive approach to measurements in a complete measurement system, it is worth taking into account programmable and stabilized laboratory power supplies. In addition, the requirements for ambient temperature measurement can be implemented by including sensors for temperature recording in the system. All this can be controlled by a single software interface to control optical, electrical and temperature measurements. This makes it easier to ensure appropriate working conditions and to prepare comprehensive measurement reports in accordance with the latest guidelines of international standards.

Unlike traditional goniophotometers, modern goniometric systems use spectroradiometers, which can be calibrated to measure absolute photometric values and to measure colorimetric values simultaneously. Due to their specific construction, LED lamps and luminaires should be measured using a spectroradiometer, which can provide appropriate, accurate measurement data for chromatic coordinates x, y, Duv index, correlated colour temperature, colour rendering indexes Ra and CRI. In addition, LED products can have different colour temperature depending on the angle of illumination, and the use of the spectroradiometer in combination with the goniometer, i.e. goniospectroradiometer, allows determining the angular uniformity of colour according to the recommendations of the CIE standards adopted by europien standardisation EN 13032-4:2015-09 and IES LM 79.

Electrical measurements

The choice of power supplies and electric meters should be dictated by the type of products to be measured. End products, ready-to-use LED luminaires or LED lamps for retrofitting require testing in conditions similar to those in which they will operate in a ready-to-use lighting installation. This usually means working with alternating current with different voltage and frequency ranges. For this reason, it is important to be able to simulate different operating environments with a single power supply.

At the LED product development stage, manufacturers are forced to test individual components individually to check their impact on the finished product. In this situation, testing will require the use of DC power supply, less often alternating voltage.

Laboratory DC power supplies are able to provide the exact current required using automatic voltage regulation, which in the final operating environment of the finished product will depend on many factors that are difficult to control.

As with standard electrical measurements, the influence of the entire electrical circuit must be taken into account. Each power cord of the test sample has its own resistance and causes a voltage drop. It is worth taking care of cable distribution, not only taking into account the safety and comfort of operation, but also the voltage drop. This requirement increases with higher current values, which can be reduced by using thicker and shorter wires with lower resistance.

The most professional approach to the problem described above consists in doubling the measurement functionality by using – apart from power supplies – also a dedicated power meter. A suitable meter can work with alternating and direct current over a wide range of voltages and frequencies with accuracy even greater than in the case of power supplies. In addition, such meters include a wider range of electrical parameters that allow you to meet the requirements of separate electrical tests. The biggest advantage of the additional use of a separate power meter is that it can measure the voltage directly at the sample, thus eliminating the impact of voltage drop on the entire supply circuit. This is where the design of modern goniometers can be used to provide additional supply and measurement circuits near the mounting plate on the goniometer column.

Additional functions

With a large number of luminaires, time is the most common problem in photometric laboratories. New designs of goniometers may allow for so-called on-fly measurements, i.e. without stopping the goniometer after setting the arm at a particular angle. In this variant, the luminaire is rotated in a given position (plane) in a uniform manner, and the fast measuring system collects a large number of measurements from the whole range. Thanks to the readings from the encoder, the computer program assigns the measured value to the preset grid of measurement steps. This requires a very fast measuring system.

For optimal use of laboratory space and ease of use, it is advisable to consider installing additional slides (rails) for smooth adjustment of the tripod with the measuring head along the measurement axis. For the measurement of small objects with a small flux, the measuring distance can be reduced without the need to set the measuring axis.

It is worth remembering that different formats of photometric files are used on the European market and beyond. Currently, the most popular are EULUMDAT (European format), IES (American format) and IEC format files used in some industries and in various regions of the world. For this reason, the software supporting measurements, reports and file formatting should be universal. It should also be possible to freely import and export photometric data and combine measurement files and photometric curves from different measurements.

Calibration of the entire goniometric system is not necessary and is not practiced. If a spectroradiometer or photometer is used, the measuring device itself must be calibrated. The manufacturer should provide a factory calibration certificate or, if requested by the customer, a calibration certificate from an accredited laboratory.

Summary

Let us remember that the choice of the right equipment is very important, but this is only the beginning. It is worthwhile to start planning the organization of the in-house laboratory in good time and take into account additional organizational and logistical aspects. We should also remember to prepare and train our staff – to that end it will certainly be helpful to choose a supplier who will not only sell the measuring equipment, but who will also be able to provide appropriate support at all stages of the process.

The next part of the article on the organization of an in-house measurement laboratory will describe the issues related to integrating spheres. The stabilization of luminaires before measurement and issues related to measurement and temperature stabilization of LED modules during measurements will be discussed.

Bibliography

[1] CIE 70-1987 The measurement of absolute luminous intensity distribution.

[2] EN 13032-4:2015-09 Light and lighting – Measurement and presentation of photometric data of lamps and luminaires – Part 4: LED lamps, modules and luminaires.

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